Manufacture of silicon carbide resistors



Feb. 25, 1936. A. J. THOMPSON 7 MANUFACTURE OF SILICON CARBIDE RESISTORS Original Filed Nov. 17, 1951 3 iNvEhFFoR ALMER J. THOMPSON BY MW ATTORNEY Patented Feb. 25, 1936 UNITED STATES PATENT OFFICE MANUFACTURE OF SILICON CARBIDE I RESISTORS Application-November 11,1931, Serial No. 575,609 Renewed July 6, 1935 8 Claims. (01. 201-76) This invention relates to improvements in the manufacture of silicon carbide resistors, and to a method wherein the electrical resistance of the finished element can be varied through a considerable range without altering either the character or the chemical composition of the resistor mix.

Silicon carbide resistors having very desirable electrical properties can be made by the process 10 usually known as recrystallization. In this process the elements are heated to a very high temperature under non-oxidizing conditions, and

the individual particles of silicon carbide apparently grow together by evaporation and redeposition without the addition of any permanent binding material. In Patent 1,906,853 to Hediger,

granted May 2, 1933, a process is described wherein the resistor is recrystallized by the passage of an electric current through the resistor itself, the resistor being embedded in a sand-carbon mix. In this process an element can be produced which has a comparatively low electrical resistance and possesses a positive temperature coeficient of resistance in the temperature range in which the resistor is usually operated. The electrical resistance of the finished element can be accurately reproduced, but in order to meet the commercial requirements for various specific resistances it has been considered necessary to use a large number of mixes, and to vary the resistance by variations in the mix from which the resistor is made.

The accurate control of the resistance by varying the mix, especially when it is desired to increase or decrease the resistance of the element by small increments, is dimcult, and a large number of accurately prepared mixes must be kept in stock in order to meet the commercial require- 40 ments in regard to specific resistance. The present process afiords a means for varying the resistance of the elements in small increments over a considerable range, even when the original unburned elements are all made from the same 45 mix 1,906,853, a slip or slurry is prepared in which fine sand and carbon are suspended in a' liquid such as water, and the unburned elements are 50 dipped or otherwise coated with the material. The consistency of the slurry is such th t the coating is of appreciable thickness. Afte the elements are coated they are embedded in a loose mix of sand and carbon, and an electric current 55 is passed directly through the resistors until they In the process described in the Hediger Patentv are heated to a temperature sufllcient to eflect'recrystallization.

Although the slip or slurry may contain the v same ingredients as the surrounding mix of loose sand and carbon, the results are entirely difierent 5 when such a coating is applied to the unburned rod, in comparison with those which obtain when the rod is merely buried in the loose mix. The slurry forms a cocoon" or protective envelope around the element during recrystalliza- 10 tion, and the surface of the rod remains intact, whereas if the rod is merely embedded in the loose mix of sand and carbon, the surface is badly pitted, the bedding material adheres to the rod,

and the diameter of the rod does not remain 5 constant throughout its length.

Heretofore, the slurry has been employed for protective purposes, the composition being maintained at an approximately constant value ior I elements of all specific resistances, the sand and 20 carbon being approximately in the proportions to form silicon carbide. I have found that the electrical resistance of the finished element depends to a large extent upon the composition 01. the slurry or coating, and that by varying this 25 composition instead of maintaining a constant sand to carbon ratio, the electrical resistance of the finished element can be varied over a considerable range by comparatively small increments. This method of control eliminates the 30 necessity of using a differentresistor mix for each specific resistance desired in the finished element, and with only a very few mixes, the entire range of specific resistances desired commercially can be produced. 35

The method of carrying out my process and nature of the results obtained will be evident from the accompanying drawing.

In the drawing:

Figure 1 is atriangular percentage composition 40 diagram in which the specific resistance of the finished heating element made from one particular mix is plotted in relation to three possible components for the coating or slurry;

Figure 2 shows a burning bed which can be used for making resistors and controlling the specific resistance in accordance with my process.

In making up the slurry for the resistors, I prefer to use materials which, under the conditions of curing, will result in the formation of silicon carbide. Silicon carbide itself may also be used as an ingredient. The variation in the resistance of the element is produced by varying the ratio of conducting to a non-conducting material. Silica can be used as one of the com- 2 1 A ponents of the slurry, and as a conducting material, either carbon or powdered silicon can be used. As the silica content is increased, the resistance of the finished element decreases, but

' as either the carbon or the silicon content is inelement ponderance of conducting material in proportion to silica, a portion of the carbon can be replaced by silicon and the ingredients will still. be in the theoretical proportion to form silicon carbide,

although the specific resistance of the finished element will be entirely different from that which obtains when sand and carbon alone are used in the usual silicon carbide ratio. With the mix used in Figure 1, the specific resistance of the element can be varied from .15 to .40 ohm per cubic centimeter (the higher resistances being from two to three hundred per cent of the 1owermost values) by merely varying the composition of the slurry, other conditions such as the, mix, 5 power input and time 01' burning being kept constant.

In making the resistor, any of the usual resistor mixes employed in the recrystallization process can be used. As an example, silicon carbide grain of the desired grit-size can be mixed with a small amount of carbon (for instance 21:0 4 per. cent) and moistened with a small quantity of an agglutinant such as a solution of sodium silicate. After molding, the resistors are dried or given a preliminary baking at approximately 600 C. The slurryis applied by dipping or by any other suitable method, and the rods are then ready for burning. In dipping the elements, the slurry is made so as to have a consistency of a thin paste, so that on dipping the resistors therein, a coating of appreciable thickness adheres to the rod.

The elements can be recrystallized in a burning bed of the type shown in Figure 2, which consists of a refractory trough I, with electrodes 2 at each end. The bottom of'the trough is lined with a sand-carbon mix 3 in approximately the proportions of 80 partspf silica to 20 parts of carbon. The coated resistors are placed carefully on the sand carbon bedding material in an end to end relationship, and are joined to each other and to the electrodes by means of small graphite blocks 5 and a paste 8 made from graphite, powdered silicon and water. The resistors are then covered with a small quantity of the sand-carbon bedding material, and an electric current passed through the elements to efiect recrystallization. A high the burning process continues. The power input 7 I is gradually increased'untll the desired maximum value is attained (as for example, 8 kilowatts per linear foot for a /z" diameter element) and the power input is kept at this value for a period of from 8 to 12 minutes, although larger diameter elements will require a somewhat different power input and time of burning. This method-of cur- ,the said conducting material in the slurry applied aosaovv ing the resistors is essentially that described in the patent of Hediger previously mentioned.

In regulating the proportions of the ingredients in the slurry, I have found a relationship between 'the electrical conductivity of the slurry andflthe resistance of the finished element. The conductivity of the resistor is to a large extent a function of the current passed through it dur-' ing the recrystallization process. For example, if an element is recrystallized in a sand-carbon 10 mix without the passage of any current through it, the electrical resistance of the recrystallized element is very high and may be of the order of magnitude of several hundred ohms, even though it is well recrystallized, whereas in passing the 5 current through the element during curing, as in the process described in the Hediger patent mentioned above, this resistance is reduced to the order of magnitude of a very few ohms. By increasing the conducting'ingredients of the slurry, go a smaller proportion of the current used for heating the element passesthrough the body of the rod than would be the case when a poorly conducting slurry is used. Even when silicon is used as an ingredient of the slurry, in which case it 5 would be expected that the. silicon would vaporize and decrease the resistance of the element, the resistance is actually increased instead of being Having thus described my invention, 1' define'it so as being within the scope of the following claims: M

1. The steps in the process of manufacturing silicon carbide resistors which comprise coating the resistors with a slurry of silica, carbon, and silicon, and subsequently curing the resistors in 5 a sand-carbon mix, by passing an electric current through the said resistors.

2. In a method of manufacturing silicon carbide resistors wherein a slurry is applied to the resistors before curing, the steps which comprise 40 forming a slurry containing silica and a conducting material which is a chemical constituent ofsilicon carbide, adjusting the ratio of the silica to the said conducting material in the slurry to produce the specific resistance desired in the fin- 45 ished resistors, applying the slurry tothe outer surface of the resistors, and curing the resistors by passing an electrlccurrent through the said resistors.

3. In a method of manufacturing silicon carso bide resistors wherein'a slurry-is applied tothe resistors before curing, the steps which comprise forming a slurry containing silica and carbon, adjusting the ratio of the silica and carbon in the slurry toproduce the specific resistance desired 55 in the finished resistors, app yin the slurry to the outer surface of the resistors, and curing the resistors by passing an electric current through the said resistors.

4. The method of producing silicon carbide reso sistors having different specific resistances which comprises app y n to the resistors a slurry containing silica and a conducting material which is a chemical constituent of silicon carbide, curing the resistors by an electric current 65 through the said resistors, and producing separate batches of resistors having different specific resistances by changing the ratio of the silica and to the said batches.

5. The method of producing silicon carbide resistors having different specific resistances which comprises forming a coating on the surface of the resistors by applying arslurry containing silicon carbide forming-ingredients, curing the 75 2,082,077 resistors by passing an electric current through the said resistors, and producing resistors having different specific resistances by changing the electrical conductivity of the respective coatings ap plied to separate batches of resistors, the change in the electrical conductivity of the coatings being produced by varying the proportions of the said silicon carbide forming ingredients in the slurry applied to the said batches of resistors.

6. In the curing of silicon carbide resistors by the passage of an electric current therethrough,

the steps which comprisepa'ssing a portion of the heating current through the body of the resistor and another portion of the current through a conducting material adjacent the surface of the resistor, and producing resistors having dif-- ferent specific resistances from uncured resistors of the same composition, the said alteration in 'resistance being effected by changing with re- 7. In the curing of silicon carbide resistors by the passage of an electric current therethrough, the steps which comprise passing a portion of the heating current through the body of the resistor and another portion of the current through 5 a conducting material adjacent the surface of the resistor, and adjusting the electrical conductivity of the material adjacent the surface of the resistor so as toobtain a resistor having a pre-determined specific resistance. 10

8. In the manufacture of silicon carbide resistors, the steps comprising providing a current carrying path adjacent and around the resistor and having a specific resistance of predetermined relationship to that of the resistor to be cured 15 therein and passing a heating current through said resistor and said adjacent path, whereby the current divides according to the respective conductivities and produces a resistor of predetermined resistance.

ALMER J. THOMPSON. 

